Titanium nitride thin film formation on metal substrate by chemical vapor deposition in a magnetized sheet plasma source

ABSTRACT

A procedure for the synthesis of titanium nitride (TiN) thin films on metal substrate by vapor deposition using a magnetized sheet plasma source is disclosed. TiN films on metal substrate exhibiting the stoichiometric TiN and Ti 2 N were synthesized in a mixed N 2 /Ar plasma with initial gas filing ratio of preferably 1:3 under the following conditions: total initial gas filing pressure of at least about 40 mTorr, plasma current in the range of about 2A to 3A and plasma discharge potential in the range of about 125V to about 150V.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. national stage application under 35U.S.C. 371 of International Application No. PCT/PH02/00003, filed Feb.27, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method of film deposition. More particularly,the invention relates to a titanium nitride film chemical vapordeposition process on metal substrate using a magnetized sheet plasmasource.

2. Description of the Related Art

Interest in new coatings and surface treatment methods has been on theupsurge during the last decade of the 1990s, especially for titaniumnitride (TiN), gauging from the number of papers/patents published onthis ceramic material. Titanium nitride films prepared on silicon arewidely used as diffraction barrier layers in large-scale integratedcircuits. A TiN film is also a remarkably hard and wear-resistantcoating on tools since it decreases the rate of abrasive wear during thecutting process as well as the chemical interaction between the tool andthe work piece because of its chemical inertness. TiN is a very stablecompound that enhances the pitting resistance of many substratematerials in most environments. Several techniques such as chemicalvapor deposition, physical vapor deposition, ion plating, ionbeam-assisted deposition, sputtering, and hybrid processes have beenused to prepare TiN films. The films produced by these techniques,however, often exhibit poor adhesion to the substrate. Coupled with thisproblem is the requirement of a high deposition temperature (>500 C) forthe effective formation of the TiN film and the relatively long durationof time required for thin film formation. In many of these techniques,the plasma dimensions, and subsequently, the substrate size also limitthe process of film deposition.

SUMMARY OF THE INVENTION

The present invention, in one broad sense, is the discovery thattitanium nitride film can be synthesized on metal substrate using amagnetized sheet plasma source.

There is a need in the art for a deposition process where a) there is noheating mechanism introduced, b) the synthesis of the film is relativelyshort, and c) the synthesis can be done over a wide area of substratesurface, without sacrificing the quality of the film. A modifiedmagnetized sheet plasma ion source (Plasma Sources Sci. Technol. 8(1996)pp. 416-423 and Rev. Sci. Instrum. 71 (2000) pp. 3689-3695) is asuitable contribution to the art of deposition of titanium nitride inthe present invention satisfying the foregoing advantages. Although theinvention demonstrates the capacity of synthesizing titanium nitride forsmall-sized samples, the wide area plasma could very well serve thecoating of larger samples. The relatively short duration of coating thetitanium nitride film on metal substrate without the use of heatingmechanisms makes this invention very promising for nitriding of metals.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description of the invention can be readily appreciated inconjunction with the accompanying drawings, in which

FIG. 1 is a schematic diagram of the plasma source used in the chemicalvapor deposition of the TiN film in accordance with the presentinvention.

FIG. 2 is a pictorial view of the gold colored TiN deposit on metalsubstrate

FIG. 3 is a characteristic x-ray diffraction scan of a (200) phase TiNdeposited film and a (220) phase of Ti₂N deposited film on metalsubstrate.

FIG. 4 is a characteristic Raman scan of the deposited TiN film (onsample D) showing the frequency shift at 267 cm⁻¹.

FIG. 5 is a characteristic Raman scan of the deposited TiN film (onsample H) showing the frequency shift at 267 cm⁻¹.

FIG. 6 is a characteristic Raman scan of the deposited TiN film (onsample I) showing the frequency shift at 267 cm⁻¹.

FIG. 7 is a microanalysis of the deposited TiN film (on sample H) byenergy dispersive x-ray analysis showing the peak corresponding to theemission of TiK_(α) radiation at 4.6 eV confirming the presence oftitanium.

DETAILED DESCRIPTION OF THE INVENTION

The teachings of the present invention can be readily understood withreference to the accompanying figures, in which details of the preferredmanner of practicing the present art are described. Accordingly, personsof skill in the appropriate arts may modify the disclosures of thepresent invention but still obtain the favorable results describedherein. Since the plasma source and its discharge characteristics arekey to the deposition process, a description of the same is in order.

Referring to FIG. 1, the plasma source is composed of five main partsnamely: the production chamber, shown as Part A in the diagram, thefirst and second plasma limiters, referred to as Parts B and C,respectively in the diagram, the main discharge vacuum chamber, referredto as Part D in the diagram, and the anode or end target, referred to asPart E in the diagram. Subsequent parts referring to FIG. 1 are numberedcorrespondingly as the description proceeds.

In this particular source used for the invention, the production chambercomprises of a single tungsten wire connected to a chuck of electricalfeedthrough rods (10) by stainless steel screws. The filament is used asthe source of ionizing electrons by thermionic emission when connectedto a power supply V3 in FIG. 1. A stainless steel assembly (20) connectsthe electrical feedtrough to the main body of the production chamber.The production chamber also comprises an enclosing chamber ofcylindrical glass (30) to provide better plasma confinement within theproduction chamber and insulator between parts (20) and (50). A port in(20) comprises a low-leak valve for a gas feed (40). The steel assembly(20) connecting the electrical feedthroughs are water-cooled (at leastabout 16 C) to withstand thermal expansion and protection of rubberO-rings connecting the glass chamber to (20) and (50).

The electron emission of the tungsten cathode is characterized by adiffusion equation that expresses the relation between the radialelectron diffusion coefficient D_(⊥) across the magnetic field B and theaxial electron diffusion coefficient D

along B,

$\begin{matrix}{\frac{D_{\bot}}{D_{\coprod}} = {\frac{1}{1 + ( {\omega\;\tau} )^{2}} = \frac{1}{1 + ( \frac{\lambda_{en}}{r_{ce}} )^{2}}}} & (1)\end{matrix}$where ω, τ, λ_(en) and r_(ce) are the electron cyclotron angularfrequency, electron-neutral collision time, electron-neutral collisionmean free path and electron cyclotron radius, respectively.

For the case of hydrogen gas, λ_(en) and r_(ce) are well known andapproximated by the relations,λ_(en)≅2.0×10⁻² /P,  (2)r _(ce)≅3.4√{square root over (T _(e))}/B,  (3)where P, B and T_(e) are the hydrogen gas pressure in Torr, magneticfield in gauss and electron temperature in eV, respectively. The cathodeplasma has electron temperature of in the range of about 0.5 eV to about1.0 eV from Langmuir probe measurements. Then the relation(ωτ)_(en)≅10⁻² B/P is obtained.  (4)

Equation (4) implies that the electron emission of the cathode decreaseswhen (ωτ)_(en)>1. According to Equation (4), the discharge becomesunstable for a strong magnetic field and a low pressure. At theproduction chamber (Part A) the pressure is typically in the range ofabout 0.6 Torr to about 1.0 Torr and in the range of about 10⁻¹ Torr to10⁻³ Torr at the main discharge chamber (Part D). With these conditions,the ωτ value is estimated as(ωτ)=10⁻² B _(zo)  (5)where the z-axis is taken to be along the line connecting the anode andthe cathode. Equation (5) means that when the magnetic field B_(zo)exceeds at 100 gauss, the electron diffusion across B_(zo) is suppressedeffectively. Therefore, to extract a high discharge current from thecathode, the axial magnetic field applied externally must be below 100gauss. A pair of Helmholtz coils comprising of copper wire wound arounda bobbin of fiber board provides the axial magnetic field.

Paschen's law states that a minimum voltage of the pre-discharge must befound for a pressure P and a distance d between the cathode and anode.In this plasma source, two sub-components comprising of a circularferrite permanent magnet (50), which is the first plasma limiter, and acoreless magnetic coil (60), which is the second plasma limiter, alsoact as the first intermediate electrode and the second intermediateelectrode for the plasma production, respectively. Initially, the firstintermediate electrode (Part B in FIG. 1) acts as the anode in thepre-discharge at the cathode region. Here, plasma is created with thebreakdown of hydrogen gas usually at a high initial gas filling pressure(at least about 1.0 Torr).

The gap between the cathode and the first intermediate electrode is atmost about 1.5 cm. An insulator separates them. This geometry is chosento satisfy Paschen's law and ensure a sufficient volume of ionizingelectrons from the cathode to be emitted into the main chamber. Thesecond intermediate electrode (Part C in FIG. 1) is axially displaced byat most about 1.5 cm from the first intermediate electrode. An insulatoralso separates the first and second intermediate electrodes. While ithouses the coreless magnetic coil (60), the second intermediateelectrode also serves as an auxiliary electrode in the main chamberdischarge after the pre-discharge initiation. By combining the twoelectrodes consisting of a circular ferrite magnet and a corelessmagnetic coil, respectively, the ion source becomes stable and ensuresuniform current density. The combination effectively reduces themagnetic field abruptly near an ion current extraction electrode.

The circular ferrite permanent magnet (50), which is enclosed in thefirst intermediate electrode, has a mean diameter of 7.0 cm The corelessmagnetic coil (60) enclosed in the second intermediate electrode is madefrom an insulated copper wire wound on a brass bobbin and operated at acoil exciting current of at most about 30 A, giving rise to at most theequivalent of 1260 A turns.

The magnetic field distributions give rise to an abrupt decrease in thedischarge anode and increases in the first intermediate electrode. Inthe production region (Part A) a reversed magnetic field due to thecombination is expected. This reversed field is very useful for cathodeprotection from backstreaming ions. The apertures leading from the twoelectrodes are covered with molybdenum for heat resistance and aredesigned so as to cause a difference in pressure between the cathode andmain vacuum discharge regions. The differential pumping that ensuresthis difference also protects the cathode. The composed magnetic fielddistribution due to the ferrite magnet and the magnetic coil has beenapproximated using three ring currents. Using the Biot-Savart law, theapproximate equation for the composed magnetic field B(Z) in gauss whereZ in cm is measured from the center of the ferrite magnet is given by

$\begin{matrix}{{B(Z)} = {\frac{787.6}{( {1 + {0.16Z^{2}}} )^{3/2}} - \frac{437.6}{( {1 + {0.049\mspace{11mu} Z^{2}}} )^{3/2}} + \mspace{85mu}{\frac{240.4}{\lbrack {1 + {0.092( {Z - 3.9} )^{2}}} \rbrack^{3/2}}.}}} & (6)\end{matrix}$

The first and second terms express the magnetic field of the ferritemagnet. The third term expresses the magnetic field of the magneticcoil.

As an indication of the validity of the approximations made,experimental values of the magnetic field distributions for the ferritepermanent magnet and magnetic coil were determined separately. Theapproximations and the actual measurements of the magnetic fielddistributions are very close, particularly at points outside of themagnets.

Using Equations (2) and (3), the following relation is obtained:

$\begin{matrix}{( \frac{\lambda_{en}}{r_{ce}} )^{2} \cong {3.4 \times 10^{- 5}( \frac{1}{T_{e}P^{2}} ){B.}}} & (7)\end{matrix}$

At the main discharge chamber the pressure P will be typically of theorder of 10⁻² Torr and the electron temperature will not exceed 9 eV fora hydrogen plasma. These values reduce Equation (7) to

$\begin{matrix}{( \frac{\lambda_{en}}{r_{ce}} )^{2} \cong {0.15{B^{2}.}}} & (8)\end{matrix}$

This relation provides an upper limit for the radial component of themagnetic field that would give a uniform electron density. This meansthat for values of B less than about 2.6 gauss, the ion current densitybecomes radially uniform. The radial magnetic field profile B_(r)(r) ofthe combined ferrite permanent magnet and coreless magnetic coil weresimilarly obtained at the extraction region and was found to be less nomore than 1.0 gauss. The radial magnetic field distributions arerelatively weak at the extraction electrode region, which according toEquation 8 will give a uniform ion current density radially.

The main vacuum discharge chamber (Part D in FIG. 1) is composed of aT-sectioned glass cylindrical chamber (90) one end of of which connectedto the second plasma limiter (60), while the other end is connected tothe anode (130). The cylinder comprises further an accessible port (140)leading to vacuum pumps and another port where the substrate holder(100) is connected.

The diffusion-pumped vacuum system is connected to the main chamber(Part D). It consists of a rotary pump as roughing pump, a mechanicalpump and an oil diffusion pump. To isolate the vacuum chambers from themechanical vibration due to the pump, a metal bellows is connectedbetween the main discharge chamber and the pump. The attainment of highvacuum of at least about 10⁻⁶ Torr served as base pressure for thechambers before gas is fed at the production chamber (Part A). Hydrogenor argon gas is fed through a low-leak needle valve in the cathodeassembly while the reactive gas (usually nitrogen) is fed by a similarlow-leak needle valve (110) at the main discharge chamber. Because ofthe construction geometry, a circular aperture in the limiters diameterat most 1.0 cm creates a pressure gradient between the cathode chamber(high-pressure chamber) and the main discharge chamber (low-pressurechamber. The difference in pressures in chambers Part A and Part D is amost an order of two.

The anode (130) or the end target (Part E in FIG. 1) serves as plasmabeam dump and is a water-cooled stainless steel hollow cylinder. Atitanium (120) is attached to the anode (130).

Going back to FIG. 1 the schematic diagram indicating the circuit usedin generating the plasma is also indicated. The plasma is generatedinside the production chamber (Part A) by thermionic emission of asingle tungsten wire using a 150V dc power supply (V3 in FIG. 1). Thefilament potential varied in the range of about 14V to about 18V at acorresponding current in the range of about 20 A to about 23 A. Thetungsten filament is negatively biased with respect to the secondintermediate electrode (limiter 60 in FIG. 1). V2 supplies the potential(in the range of about 60V and about 70V) at the production chamber at acurrent in the range of about 3 A to about 4 A. The potential in therange of about 125V and about 150V at the main chamber (from the limiter60 to anode 130) is supplied by the source V1 giving rise to acorresponding current in the range of about 0.8 A to about 1.5 A. Aswitch S2 connected to another switch S1′, which in turn is connected toanother switch S1 provide a switching sequence process in the circuitfrom the cathode to the anode. Steady-state argon plasma is normallyproduced in the main chamber using this process. The plasma form iscylindrical has a diameter of at least about 1.5 cm and at least about30 cm long from the limiters to the anode/end target.

The transformation of the cylindrical plasma is done by arranging tworectangular strong permanent magnets (at least about 1.5 kGauss on thesurface) on opposite sides of the plasma column as shown in FIG. 1 (80).The z-axis is taken along the plasma column, the y-axis perpendicular tothe permanent magnets and the x-axis parallel to the permanent magnetsin the cross-section of the plasma volume. The magnets are set at mostabout 7.0 cm away from (60). The permanent magnets are positioned withtheir magnetic polarity as shown in FIG. 1 (60). With the magnets inplace, a new magnetic cusp field with components B_(x), B_(y), B_(z)along the x-axis, y-axis, and z-axis, respectively, is added to theinitial magnetic field B_(zo). The B_(x) components are meant to expandthe plasma column along the x-direction and the B_(y) components cancelout in the x-y plane. The direction of the initial magnetic field B_(zo)is taken to increase the field intensity towards the discharge anode(130) and to decrease it towards the second intermediate electrode (60).The combined magnetic field distribution, due to B_(zo) and the twopermanent magnets with components (B_(x), B_(y), B_(z)), effects thechange of the cylindrical plasma into a sheet plasma. An adjustment inthe width of the sheet plasma is effected through a relation betweenB_(x) and (B_(zo)+B_(z)). Similarly, an adjustment of the thickness ofthe sheet plasma if effected through a relation between B_(y) and(B_(zo)+B_(z)).

In carrying out the invention, several procedures were conducted in theoperation of the magnetized sheet plasma source.

Evacuation of the chamber was done with a preferably a 500-l/m back-uprotary pump coupled to preferably a 10.16 cm oil diffusion pump.Pressures were monitored by ionization and Pirani gauges. Base pressurewas usually in the order of at least 1.0×10⁻⁶ Torr. Argon gas is fedthrough a slow leak needle valve (40) in the production region while asimilar low leak needle valve (110) allows the reactive gas to be fed inthe deposition chamber. Because of the construction geometry, a circularaperture of diameter at most about 1.0 cm in the limiters creates apressure gradient between the production chamber (high-pressure chamber)and the deposition chamber (low-pressure chamber).

Gas pressures quoted here refer to pressures in the deposition chamber.The vacuum sensors were calibrated for nitrogen. The pressure readingwas not corrected for the gas sensitivity factor for argon.

The sheet plasma form includes fast electrons within the core plasma ofseveral millimeters and dimensions at least about 21.10×13.40 cm²accompanied by cold diffused plasma electrons at the periphery.Energetic electrons at the center of the sheet plasma havingtemperatures of at most about 25.0 eV are detected by using a singleLangmuir probe for a 3.5 A plasma current. Under the same conditions,negative ions of titanium having ion energies to be at most about 20.0eV are obtained using a potential-type electrostatic energy analyzer.

Clean metal substrates with dimensions of at least about 1.1×1.1×0.05cm³ were used in the process, although the sheet plasma dimensions makeit possible for wider area applications. The substrate is placed on awater-cooled holder (100) (preferable of temperature of at most about10C) which is connected to an adjustable bellows enabling the sample tobe positioned anywhere from the core of the sheet plasma to its outerperiphery. It was determined that deposition could easily be conductedif the substrates were immersed in the core of the sheet plasma. Thesubstrate is placed such that its plane is parallel to the plane of thesheet plasma. Provision is also made for biasing the substrate. Furtherdischarge cleaning of the substrate was done in pure argon plasma for afew minutes.

Several samples were prepared under varying conditions of discharge(preferably in the range of about 2A to about 3A plasma current) anddeposition times preferably in the range of at least 10 minutes to about20 minutes. All samples prepared were immersed in the core of the sheetplasma. Argon pressure was set at preferably about 30 mTorr whilenitrogen constituted 25% of the total gas filling pressure of at mostabout 40 mTorr. After deposition the samples are carefully extracted andstored for examination by X-ray diffraction (XRD), Raman spectroscopyand energy-dispersive X-ray emission (EDX) spectroscopy.

The color of the deposit is initially used as indicator of the presenceor absence of TiN deposit on the substrate. TiN has a characteristicgold color. As an example of the deposition run, samples identified asD, H and I seemed likely to have TiN film deposits. These samplesexhibited the characteristic yellow-gold color shown in FIG. 2. Sample Dwas obtained under the following conditions: plasma current of at leastabout 3.0 A, no bias potential and deposition time of at most 15minutes. Sample H had the following conditions for deposition: plasmacurrent of at least about 3.0 A, bias potential of at most about 250Vand deposition time of at most 15 minutes. Sample I had the followingconditions: plasma current of at least about 3.0 A, target biaspotential of at most about −250V, and deposition time of at most about20 minutes. The film color for this sample was gray. The titanium targetpotential did not seem to have any effect on the color of the depositedfilm. Without the bias potential, energetic electrons sputter thetitanium target as it assumes the anode potential. With the negativebias, however, argon ions sputter the titanium target. The eventualbreakdown of the film at prolonged deposition times may be a result ofsputtering.

The deposited films were first confirmed by XRD. The XRD scan for sampleI is shown in FIG. 3. The peak at 43° in FIG. 4 corresponds to the (200)phase of TiN. The adjacent peak prominent at 44.68° is attributed to theα-Fe (110) phase of the substrate. The broad peak is attributed to thelattice mismatch of the deposited TiN film and the steel substrate. Theother significant peak that appears at 49.95° in FIG. 3 corresponds tothe (220) phase of Ti₂N. This sample is exposed to the plasma for arelatively longer time. The deposited TiN have been dissociated bysputtering and then recombined to form this additional phase. Suchformation has been observed to be more favorable for longer timedeposition. The XRD pattern for the unbiased sample (D) has a the peakhaving the same as that of sample H at 2θ=43° associated with the TiN(200) phase.

The Raman spectra typical of samples D, H and I are shown in FIGS. 4, 5and 6 respectively. The frequency shift at 267 cm⁻¹ is very pronouncedfor these sample. This corresponds to the calculated value foracoustical phonon scattering for stoichiometric TiN. The presence of thehigh frequency shift (at optical range) at about 370 cm⁻¹ for sample I(FIG. 6) confirms the nonstoichiometric nature of the TiN film for thissample. For this sample, the relatively longer deposition time resultsto other TiN phases. In this case, TiN_(0.995). The absence of the highfrequency shift at 370 cm⁻¹ for samples D (FIG. 4) and H (FIG. 5) is anindication of the stoichiometric nature of the produced TiN film onsteel under the conditions of the experiment.

Results of microanalysis using EDX is shown in FIG. 7 for sample H. Thepeak corresponding to the emission of TiKα radiation at 4.6 keV confirmsthe presence of Ti. Other dominant peaks in the figures are thoseassociated with emissions due to metal constituents of the substrate.

Electron micrographs of sample H shows sheet-like structures indicativeof a more uniform film formation unlike the case of a columnar type ofgrowth.

CONCLUSION

A magnetized sheet plasma source designed for volume-production ofnegative hydrogen ions has been slightly modified by placing a titaniumdisk as target placed at the anode. Argon has been employed assputtering gas for the titanium target. Nitrogen introduced as reactivegas in the main chamber at preferably 25% of the initial total gasfilling pressure of at most about 40 mTorr. Relatively low plasmacurrents of at least about 3.0 A were found suitable for the productionof TiN in the plasma-enhanced chemical vapor deposition process. Atthese currents, more argon ions are produced to sputter the titaniumtarget thereby increasing the probability of producing the TiN film. Theresults of the XRD and Raman scans confirm the synthesized TiN filmsunder these conditions.

1. A method for the synthesis of titanium nitride on a metal substratein a plasma enhanced chemical vapor deposition chamber comprising:introducing a discharge gas into a production chamber through a low leakvalve, the production chamber having a cathode with a tungsten filament;igniting the discharge gas in the production chamber by thermionicemission of the tungsten filament using a first DC power supply, theignition of the discharge gas producing a gas plasma discharge;supplying a potential to the production chamber from a second DC powersupply; stabilizing the gas plasma discharge with a first plasma limiterand a second plasma limiter, wherein the first plasma limiter has afirst magnet that acts as a first intermediate electrode, wherein thefirst intermediate electrode acts as a first anode, wherein the secondplasma limiter has a second magnet that acts as a second intermediateelectrode, wherein the tungsten filament is negatively biased withrespect to the second intermediate electrode; igniting the gas plasmadischarge in a main vacuum chamber using a third DC power supply togenerate a potential between the second intermediate electrode and asecond anode, the ignition of the gas plasma discharge producing acylindrical plasma, the main vacuum chamber having a movable substrateholder; converting the cylindrical plasma in the main vacuum chamberinto a sheet plasma via a third magnet and a fourth magnet, the thirdmagnet and the fourth magnet having opposing fields that are separatedby a distance; sputtering a titanium target attachable to the secondanode by the gas plasma discharge; introducing a reactive gas to themain vacuum chamber, the introduction of the reactive gas producing amixed plasma; and exposing the metal substrate to the mixed plasma. 2.The method of claim 1, wherein said discharge gas is preferably argon.3. The method of claim 1, wherein said reactive gas is nitrogen.
 4. Themethod of claim 1, wherein the production chamber has a base pressure ofat least about 10⁻⁶ Torr and the discharge gas is introduced to theproduction chamber at a pressure of at least about 30 mTorr.
 5. Themethod of claim 1, wherein the tungsten filament has a potential in therange of about 14V to about 18V and the tungsten filament has a currentin the range of about 20 A to about 23 A.
 6. The method of claim 1,wherein the potential supplied by the second DC power supply is in therange of about 60V to about 70V and the current is in the range of about3 A to about 4 A.
 7. The method of claim 1, wherein the potentialsupplied by the third DC power supply is in the range of about 125V toabout 150V and the current is in the range of about 0.8 A to about 1.5A.
 8. The method of claim 1, wherein the third magnet and the fourthmagnet are (at least about 1.5 kG on the surface, and wherein the thirdmagnet and the fourth magnet are separated by a distance of at leastabout 10.0 cm outside a chamber wall just after the second intermediateelectrode.
 9. The method of claim 1, wherein the substrate holder isconnectable to an adjustable bellows enabling the substrate holder tomove perpendicular to a plane of a core of the sheet plasma to the outerperiphery to immerse the metal substrate in the core of the sheetplasma, wherein the substrate holder is susceptible to biasing.
 10. Themethod of claim 1, wherein the reactive gas is introduced to the mainvacuum chamber at a ratio of one part to three parts discharge gas in atotal initial gas filling pressure of at least about 40 mTorr.
 11. Themethod of claim 1, wherein the metal substrate is exposed to the mixedplasma for a period in the range of about 10 minutes to about 20minutes.
 12. The method of claim 1, wherein the ignition of the gasplasma discharge in the main vacuum chamber is through a sequentialswitching process utilizing three connectable switches in between thecathode and the second anode.